This invention relates to high temperature, secondary electrochemical cells and batteries of such cells that can be employed as power sources for electric automobiles, hybrid electric vehicles or for the storage of electric energy generated during intervals of off-peak power consumption. It is particularly applicable to electrochemical cells that employ molten electrolytic salts as an electrolyte and metal oxides, sulfides or sulfur as positive electrode reactants. Examples of cells of these types are presented in U.S. Pat. No. 3,907,589 to Gay et al., entitled "Cathode for a Secondary Electrochemical Cell"; and in U.S. patent applications Ser. Nos. 555,317 to Steuenberg et al. entitled "Improved Cathode Composition for Electrochemical Cell"; and Ser. No. 510,840 to Yao et al., entitled "Electrochemical Cell Assembled in Discharge State". Each of the above are assigned to the assignee of the present application.
In these previous cells lithium, sodium, or lithium-aluminum alloys have been proposed as negative electrode active materials. Some of the cells operate with lithium or sodium metal in molten state at a sufficiently high temperature to provide a molten electrolytic salt. However, the use of molten metals or molten metal alloys as active materials presents difficult engineering problems. Special procedures have been required to retain the molten material within the electrode, for example, see allowed U.S. patent application Ser. No. 549,635, to Vissers et al., entitled "Improved Anode for a Secondary, High-Temperature Electrochemical Cell". Also, electrical shorting and materials attack have resulted from the molten active materials.
Other cells employing solid lithium-aluminum alloys as electrode material have been prepared and operated with considerable success. However, lithium is a scarce and expensive element while calcium and sodium are more abundant. By employing electrodes and electrolytes that predominately include calcium, its halides and the halide salt of sodium, electrochemical cells of potentially reduced cost can be provided.
Various cells have been proposed that employ calcium metal as a negative electrode reactant. These cells are generally primary cells that are not rechargeable. For instance, thermally activated, primary cells have used calcium as active material with alkali metal halides and alkali earth metal halides for the electrolytic salt. The electrolyte is originally solid, but fuses on temperature rise to activate the cell. Where lithium or sodium halides are employed as constituents within the electrolytic salt, liquid calcium-lithium or sodium-calcium alloy can be produced by reaction between the electrolyte and the solid calcium metal electrode. Liquid alloy formation is undesirable in secondary cells with molten electrolytes because it can lead to electrical shorting and various engineering difficulties. For example, molten alloys of such reactive metals are highly corrosive to ceramic and insulative materials and can also undergo wicking within fabric insulators and separators.